Bridge wire for producing high temperature explosion



Jan. 26, 1965 L. KOPlTO 3,167,014

BRIDGE WIRE FOR PRODUCING HIGH TEMPERATURE EXPLOSION Filed Oct. 5, 1961 l4 l2 f F I G.

2 8 @Q 2 E i l l l l 0 so 100 so 200 TIME IN m/ F l G. 2

INVENTOR LOUIS KOPITO Wm 5" m ATTORNEYS United States Patent 3,167,014 BRIDGE W FilR PRODUCENG HEGH TEMPERATURE EXPLOSIQN Louis Kopito, Brookline, Mass, assignor to Baird-Atomic, Inc., Cambridge, Mass a corporation of Massachusetts 7 Filed Get. 5, 1961, Ser. No. 143,221

6 Claims. (Cl. 10228) This invention relates in general to a novel method and apparatus for producing a high temperature explosion of very short duration and more particularly concerns the use of a graphite filament or filaments or graphite areas in an exploding wire circuit for the purpose of generating a light flash of intense brilliance acsulting in vaporization of the wire accompanied by a high level output of light of very short duration and shock waves.

Heretofore, exploding wires, as a source of short lived heat, light and shock waves have been limited in their utility by reason of "the fact that the wire itself is destroyed in the explosion, and must be replaced each time the process is repeated. Furthermore, the temperature,

. shock waves and light output of exploded metal wires cannot exceed certain limits because of the melting and/ or sublimation point of metal and alloys.

ductivity decreases as temperature rises. As a result there is a lesser working limit to the amount of electrical energy that can be applied usefully to the wire.

Accordingly, it is'an object of the present invention to provide a new and improved method for generating short 'lived bursts of extremely high temperatures accompanied by shock waves and an emission of light of intense brilliance.

Another object of this-invention is to provide a novel Wire exploding apparatus which may be operated reing wire circuit in which the explodable element is graphite material fabricated i'n the form of asingle filament which may be combined with other filaments into a sold under the trade designation Graphite Cloth.

3,167,014 Patented Jan. 26, 1965 FIG. 2 is a graph on Which is plotted a time versus current curve.

Referring now to FIG. 1, a simplified system for carrying out the invention typically includes a condenser 10 charged from a power source 12 and dischargeable through an element 14. A switching device 16 and a resistor 18 completes the basic requirements for the circuit.

Operating parameters for the circuit may be selected from a very wide range of values, according to the particular results desired. By way of illustration, the

, applied voltage may be varied from a relatively low potential of 4000 volts up to voltages in the 25 kv. range. The capacitance of the condenser 10 may range from 2000 tfd. to 2000 fd. for example. Since the element 14 is normally of a very low resistance and inductance, the condenser 10 and the remainder of the circuit components should also be designed with low inductance and resistance. Thyratron tubes or triggered gap switches for example, may be employed as the switching device depending upon the particular application and conditions.

In the practice of this invention, the element 14 is fabricated from filaments or strands of a graphite material such as that available from the National Carbon Company and used in a woven or felted fabric which is This material is formed from fibers of high purity graphite carbon having a tensile strength in the 50,000 to 100,000 p.s.i. range and characterized by high thermal conductivity and good electrical conductivity. The graphite fibers may be produced by processing carbonaceous materials at temperatures up to 5400 F.

Since all metals display positive coefficients of electrical conduction, resistance of the wire increases and electrical consilicon, boron, copper, nickel and sodium. The material sublimes at approximately 6600 F. '(3650 C.) without strand or alternatively the strands may be woven into a section of cloth. A graphite. element in a circuit adapted to deliver a pulse of high electrical potential is capable of generating shockwaves'together with a light flash of greater intensity and a temperature level con- N siderably higher than that available from a similarcir cuit employing a metal explodable element such as'copper, silver or platinum.

But these and other features of the invention, along .with further objects and advantages thereof, will become more readily apparent from the following de tailed description taken in connection with the accommately 1400 individual fiberseach of a generally circular cross section and each approximately 8 to 10 microns in diameter.

The high melting point of the graphite material together with its low thermal mass, its negative coeflicient of electrical conductivity, its excellent black body characteristics and the finenessof its fibers, all contribute to produce an exploding wire effect that is several magnitudes greater than that obtainable from a metal wire when a pulse of high electrical energy is applied.

It will be appreciated thatthe ability of the graphite filament to radiate a large quantity of light is greatly enhanced by reason of the fact that the filamentis co-mposed of a large number of fine fibers. Since each fiber displays a very high emissivity rating, the total emission of the entire filament will exceed by a substantial measure the light output generated by a solid conductor having the same diameter as the graphite filament.

In practice, when the circuit is closed by means of the switch 16, a heavy flow of current of short duration passes through the graphite filament 14. As suggested in FIG. 2, the current rises at a rate of approximately 10 amp/sec, for example, and the current density increases at the rate of 10 arnp./sec.-cm. to reach a peak density of about 2 10 amp./sec.-cm. The power input for such current densities would be in the tens of megawatt range. The elapsed timeinvolved in the explosion is extremely short, typically being in the order of 50 to M sec. When a relatively low voltage about 4000 volts together with a capacitance in the neighborhood of 375 ,ufd., is applied to the graphite filament, an explosive eifect is generated without destruction of the filament. In the case 9 a a of a metal wire such a voltage input would produce an explosion of low intensity along Withdestruction of the In contrast, the graphite filament will produce a that described above may be explained'on the following basis:

A large quantity of electrical energy is delivered to the filament in a fraction of a microsecond andis converted into heat energy by making the filament the most highly resistive part of the circuit. Since the mass of the filament is quite small, the graphite will traverse 'a rapid temperature transient before the heat can escape. In this transient stage the graphite changes from ambient conditions to a vapor phase and returns to the solid state within a fraction of a millisecond. To produce the high temperature which is characteristic of these explosions it is necessary to add energy to the system at a rate that will exceed the, rate of energy loss through radiation and other'causes. Typically, temperatures in the range of 100,000 K may be obtained by exploding graphite filaments with short pulses of high voltage current.

Since the graphite filament displays-a negative coefficient of electrical conductivity, it, of course, conducts a current better as the temperature rises. In contrast, all

' metals have positive coefiicients so that their conductivity decreases with a rise in temperature. I

In designing the circuit for exploding a graphite filament considerable attention should be directed towards the switching device employed. Should a triggerable air spark gap or trigatron be used, the leads in the trigatron are mounted between two parallel plates. The rise time of a switch of this type should be in the neighborhood of 7 m sec. When a thyratron (rise time of 8 m sec., for example) is the switch, its leads are mounted between two hemispherical metal balls.

Several methods exist for determining the temperature of a solid or liquid all based on a radiation law which is true for the radiation from a blackbody furnace.

Plancks radiation law for monochromatic radiation is:

C1 ecu x211.

where c 3,7413 10 W. M2 and c =14,3ssp. K

The integral of Eq. 1

gives the total energy radiated Where This is the Stefan-Boltzmann law. Employing radiation data from two Wavelength A and M, the ratio of EM to Ek gives E \1 )\25E \1+ C2/)\1T O1/ \2T which simplifies to and tures and 1 NR plotted against 1/ T to yield a calibration curve. This calibration curve may be extrapolated over 7 where V is the potential to which the "capacitor 10 is into the Wire inwatts;

4 ranges of temperatures for which the radiometerjhas a linear response.

Another means of determining the temperature of the explosion is from the equation,

g [MC,,(T)T]=P-AT (Hydrodynamic) V where M is the mass of the filament, and C (T)'is the specific heat at constant volume. This ranges from 0.1

to 0.7 joules/g/"K for temperatures of interest- T is the wire temperature in K; P is the electrical power r=Stefan-Boltzmann constant:

- 5.7 l0- erg./sec./cm. K

and A is the area of the filament or the exploding surface.

When the filament has reached its maximum temperatrue the rate of change of internal energy would be zero. That is Thus, it is shown that the temperature is proportional to the fourth root of input power divided by the surface area of the filament so that the smaller the diameter .of the exploding element the greater the temperature'for a given amount of applied energy. 7

The maximum power that can be delivered to a resistive load in an RCLcircuit is F max;

charged and L and C represent the inductance and capacitance. This clearly shows the requirement of a high voltage capacitor with a low L/ C ratio.

-While the invention has been described with particular reference to the illustrated embodiment, it will be understood that. a variety of circuits in a variety of circuit parameters may be employed to explode the graphite filament depending upon existing conditionsand theparticular efiect desired.

Having thus described .my invention, what I claimand desire to obtain by Letters Patent of the United States is:

1. A circuit for producing an explosion of very short duration and characterized by an intense flash of light, high temperature and high velocity shock waves, comprising a graphite filament formed from a number of fine graphite fibers, power means for energizing said filament, condenser means for storingan electrical charge of'high potential and switching means for closing the circuit whereby a pulse of high electrical energy is applied to said filament.

2. A circuit according to claim 1 in which thegraphite filament is the mostresistive element.

3. A circuit for producing an explosion of very short duration and characterized by an intense hash of light,

high temperature and high velocity shock waves, comclaim 3 in which'the switch- 5. A circuit according to claim 3 in which the switching means is a thyratron device.

6. A circuit for producing an explosion of very short duration and characterized by an intense flash of light, high temperature and high velocity shock waves, com prising a graphite element formed from a number of fine graphite fibers, condenser means for storing an electrical charge of high potential and rapid rise switching rneans for closing the circuit whereby a pulse of high electrical energy is applied to said element.

References Cited in the file of this patent UNITED STATES PATENTS Beardslee Aug. 18, Windes July 11, Apstein et al. Jan. 28, Roth Nov. 25, Menke et al. Jan. 12, Scherrer Feb. 9, Atkins et al. Mar. 1, Bianchi et al. Sept. 5, 

6. A CIRCUIT FOR PRODUCING AN EXPLOSION OF VERY SHORT DURATION AND CHARACTERIZED BY AN INTENSE FLASH OF LIGHT, HIGH TEMPERATURE AND HIGH VELOCITY SHOCK WAVES, COMPRISING A GRAPHITE ELEMENT FORMED FROM A NUMBER OF FINE GRAPHITE FIBERS, CONDENSER MEANS FOR STORING AN ELECTRIC CHARGE OF HIGH POTENTIAL AND RAPID RISE SWITCHING MEANS FOR CLOSING THE CIRCUIT WHEREBY A PULSE OF HIGH ELECTRICAL ENERGY IS APPLIED TO SAID ELEMENT. 